That's a good start You could get even more expression out of those five frames though

Here's some feedback:The length of his feet are not consistent, unless you're trying to add depth by for example pointing his foot a bit towards the viewer, I would make them look the same. His right kneecap is lower than the left, from an anatomical perspective they would in most cases be horizontally aligned in the given pose.

As for the animation, you should avoid the use of extra colors. Use the colors in your palette and experiment with the pixel placement.

Here are two examples of what I've been able to achieve.Sprite-sheet in case you want to use or study it

A less dynamic one, this one probably gets closer to what's you're going for.

You could make it look more impactful and expressive using the previous technique again. Also, I don't think you gain anything from shading the muzzle flash like that, limit yourself to only one red and yellow color

edit:here's some quick visual feedback:disregard his left hand, in the version above, as it makes no sense... you would put it beneath the barrel.